As part of the frying or oil-roasting process, the frying oil degrades. Undesirable byproducts form which affect the taste, color and shelf life of the finished products. The food industry recognizes that filters which remove solids from the frying oil can prolong the life of the oil. However, some of the byproducts of the oil breakdown are soluble and cannot be removed by simply filtering the solids from the oil. Hence, active filtration adsorbents, such as magnesium silicate, have been added to the oil for adsorbing the soluble byproducts. Most of these adsorbents are in powder form and as such increase the amount of solids which in turn must be removed from the frying oil.
Solids/liquids filters are common in the food industry. Gravity filters are common which use a disposable, non-woven filter media and the weight of the oil to force the oil through the media. Vacuum filters are used which often have woven metal screens to catch the solids and create a liquid vacuum to pull oil through the screen. Centrifuges are also used to remove the solids by centrifugal action. Another commonly used type of filter is a plate and frame which has a series of vertically-oriented plates containing filter media and pump pressure is used to force the oil through the media.
Filters can be used in a batch mode or in an on-line, continuous mode. In the batch mode the fryer is stopped and the oil removed to a treatment tank. The oil is then pumped to the filter from the treatment tank for solids removal. In the continuous mode the filter takes a slipstream of the oil through it and returns it directly (or indirectly through an intermediate tank) to the fryer while the fryer is on-line in normal operation.
The key difference between the two modes is that with continuous filtration the fryer does not have to be stopped and the oil removed for a separate, off-line filtration process. Many of the industrial frying lines use conveyors to continuously move the food through the frying oil bath. This bath often is maintained at temperatures of between about 275° F. and 415° F. Stopping the fryer to remove and filter the oil is not practical due to loss of production. Instead the operator might dilute the used oil with new oil to maintain acceptable amounts of oil degradation byproducts. For example, such process may be used to keep free-fatty-acid (FFA) levels less than 1% or to keep the color of the oil from becoming too dark.
Existing filters which can be used effectively in a continuous mode are the gravity, vacuum and centrifuge filters. However, none of them have the solids-removal efficiency to prevent migration of the filter aid adsorbent through the filter to the frying oil source. Some of these agents are quite fine (by design so that they have more surface area to allow greater adsorption) and one micron level filtration is necessary to remove them. In addition, the flow rate through the filter must be sufficient to keep up with the solids added through the normal frying process or the quality of the oil in the entire fryer will degrade over time. The combined requirement of high-solids-removal efficiency with sufficient flow rate generally exceeds performance of filters currently in use.
Plate and frame filters have better solids-removal efficiency but usually are manually operated and hence not well-suited for continuous operation. Because the oil is very hot, substantial downtime is required for the press to cool off so that an operator can unload it. Such filters are better suited to batch-mode operation.
U.S. Pat. No. 6,368,648 issued to Bertram et al, discloses an Adsorbent Filtration System for Treating Used Cooking Oil. The Bertram process suffers from a number of disadvantages. For example, the filter in Bertram requires complex oil circulation controls. Importantly, the system is inefficient because it requires more than one pass of the oil through the filter before it is returned to the fryer. The system is further inefficient and complex for its manner of adding adsorbent into the system. There are numerous diverting valves, transport lines, pumps and other structure required for operation of the system. Moreover, the adsorbent is not added directly to the oil, but rather is instead indirectly added into the transport lines. Such addition of adsorbent into the transport lines is contingent upon numerous process steps, including the accumulation and detection of two separate and “pre-determined” amounts of oil in the system. For at least these reasons, the Bertram process is undesirable because it is needlessly complicated and inefficient.
More specifically, in Bertram the process begins by filling the entire system with used cooking oil. The oil is drawn from the product supply (e.g., deep fryer) and is passed through the filter and into a holding tank. A filter cake is formed by adding adsorbent into the transfer lines of the system where the adsorbent mixes with used cooking oil. The oil and adsorbent are then passed to the holding tank and therefrom through the filter where the adsorbent contacts the filter and forms a filter cake. At no time is mixing occurring in the holding tank. Importantly, when adsorbent is added into the lines to mix with the oil, no oil is being passed through the filter. After the filter cake is deposited on the filter, oil is drawn from the product source and is passed through the filter and therefrom is returned to the product supply. During the filtering process, no adsorbent is added to the used cooking oil that is being drawn from the oil source and passed through to the filter. Bertram relies exclusively on the interaction of the oil with the filter cake for the removal of FFAs and other impurities. After twenty minutes of continuous flow of untreated oil from the product source through the filter and the filter cake, the filter cake is discharged. The system then resets beginning again with filling the system with used cooking oil and forming a new filter cake on the filter.
From the foregoing description of known systems, it can be seen, therefore, that there is a need within the industrial frying arena for a system which reliably and effectively adds filter aid adsorbent and uses a filter which can remove them in a continuous mode of operation.